The present invention relates to a power supply device which includes a monolithic inverter with reduced high harmonics.
An example of conventional power supply devices of this type is disclosed in Japanese Laid-open No 149845/1996.
A power supply device disclosed in the aforementioned Japanese Patent Laid-open No. 149845/1996 includes a full-wave rectifying circuit connected to a commercial AC power supply, and a half bridge type inverter circuit which has a pair of switching elements and is connected to said full-wave rectifying circuit. A series circuit consisting of an inductor and a condenser is connected in parallel with one of the switching elements of the half-bridge type inverter circuit, while a diode and a lamp are connected in parallel with the other switching element. Another condenser is connected in parallel with the series circuit that consists of the inductor, the condenser and the diode mentioned above.
With this configuration, the fluctuation range of electric current is reduced by storing high frequency energy on the latter condenser when the output voltage from the full-wave rectifying circuit is high and supplying energy to the inverter circuit when the output voltage is low. In addition, the inverter circuit is constantly supplied with electric power from the full-wave rectifying circuit regardless of the level of output voltage from the full-wave rectifying circuit. As current is thus fed from the commercial AC power supply without interruption, distortion of input current is reduced. Thus, the above configuration is capable of reducing high harmonics and current distortion.
The circuit described above in Japanese Laid-Open Patent No. 149845/1996 can readily be used in a half-bridge type inverter circuit, since one of the two switching elements is always in the `ON` state. However, in applications where only a single switching element is present, there is a period during which the switch does not conduct, or is in the `OFF` state. During this `OFF` state, a controlled resonance circuit functions independently of the switch, resulting in a free oscillation state. This free oscillation state limits the scope of power inverter type applications available for the circuit disclosed in Japanese Laid-Open Patent No. 149845/1996.